A model for the interaction of 6-Lauroyl-2-(N,N-dimethylamino)naphthalene with lipid environments: Implications for spectral properties

Although 6-lauroyl-2-(N,N-dimethylamino)naphthalene (LAURDAN) is now widely used as a probe for lipid systems, most studies focus on the effect of the lipid environment on its emission properties but not on the excitation pro perties. The present study is intended to investigate the excitation proper ties of LAURDAN in diverse lipid environments. To this end, the fluorescenc e properties of LAURDAN were studied in synthetic ester and ether phosphati dylcholines and sphingomyelin vesicles below, at and above the correspondin g lipid main phase-transition temperature. The excitation spectra of LAURDA N in these environments always show at least two well-resolved bands. In th e different lipid vesicles the behavior of the red band in the LAURDAN exci tation spectra is sensitive to the lipid chemical environment near the prob e fluorescent moiety and to the packing of the different lipid phases (gel and liquid crystalline). We propose that the interaction between the LAURDA N dimethylamino group and the ester linkage of ester phospholipids is respo nsible for the strong stabilization of LAURDAN's red excitation band in the gel phase of ester phospholipid vesicles. We discuss the consequence of th ese proposed ground-state interactions on LAURDAN's emission generalized po larization function, In the context of variable excitation wavelengths, inf ormation concerning solvent dipolar relaxation through excitation generaliz ed polarization function is also discussed.